1. Field of the Invention
The present invention relates to a range detection optical system used in photographic cameras, cinematographic cameras, video cameras, etc.
2. Related Background Art
Active distance-measuring optical systems employ the theory of triangulation. Namely, an optical system of such a type is provided with a light projection system and a light receiving system the optical axes of which are separated at a predetermined distance (base length) from each other, as being so arranged that the light projection system projects an infrared beam toward an object and the light receiving system receives a reflection beam returning from the object. The optical system employs a method for measuring a distance between a predetermined position and the object (hereinafter referred to as "range detection"), based on an amount of deviation along the direction of the base length between the optical axis of the light receiving system and a position where the reflection beam impinges on a photodetector in the light receiving system. The active method is excellent in that the range detection can be made independent of the contrast of the object.
The conventional active range detection optical systems, however, had a problem that object-side numerical apertures of a light receiving lens for the object in a far range were smaller than those for the object in a near range. This resulted in decreasing a quantity of light received by the light receiving system, which could in turn cause an error in range detection.
If the object is in a far range an enlarged image of a light emission source is projected onto the object. Thus, a defocus amount was great in the far range, and the projected image was larger than the object. This caused the light receiving system to fail to receive most of the projected light except part, which could cause an error in range detection.
As described, the conventional active range detection optical systems had lower accuracy of range detection for far objects. Japanese Patent Application Laid-open No. 55-88001 discloses an aberration correction method for light projection lens and light receiving lens, solving the above problem to some extent and improving the range detection accuracy.
Further, Japanese Patent Application Laid-open No. 1-158409 discloses a range detection optical system in which each of the light projection lens and the light receiving lens is composed of two lenses to correct the image plane.
In the range detection optical system as disclosed in the Japanese Patent Application Laid-open No. 55-88001, the light projection system is so arranged that the light emission source is located at the light-source-side focal point of the light projection lens, and, therefore, the image of the light source through the light projection lens is at infinity. This arrangement can lower the defocus amount of the projected image if the object is located considerably far away from the light projection lens. Conversely, if the object is located near the light projection lens, the defocus amount of the projected image increases so as to lower the range detection accuracy down to an insufficient level, which has been inconvenient.
In the light receiving system the photodetector is located at the detector-side focal point of the light receiving lens. If the object is in a near range, the position of an image of the object through the light receiving lens greatly deviates from the photodetector, so as to blur the image on the photodetector, which could inconveniently cause an error in range detection.
Further, since the range detection optical system as disclosed in Japanese Patent Application Laid-open No. 1-158409 is so arranged that each of the light projection lens and the light receiving lens is composed of two lenses, a reduction of production cost would be difficult. Another problem was that the transmittance was degraded without an anti-reflection coat on lens surfaces.